1. Field of the Invention
The present invention relates to electronic systems for generating special effects in color TV and color motion pictures and, more particularly, to a system using the luminance portion of one image to key in a second image to form a composite video picture.
2. History of the Prior Art
There presently exists the capability of keying out a portion of one image and replacing it was part or all of another image in television and motion picture systems. The "blue screen traveling matte" process, in various forms, has been known and successfully utilized by the motion picture industry for many years in the creation of special photographic effects. An electronic blue screen system known commercially as chroma key has, in spite of certain limitations, been widely used in commercial television production.
All traveling matte systems have certain common characteristics--the action phase is photographed against a colored backing or screen of high saturation, producing what is known as the foreground scene. By photographic (or, in the case of chroma key, electronic) systems and methods the color backing is removed from the foreground scene and is replaced by a background scene which is photographed separately. The final picture, therefore, contains (a) the foreground action that is photographed against the color backing, and (b) the background scene, combined photographically or electronically.
One example of a chroma key system is provided by U.S. Pat. No. 3,678,182, Boxman et al, Chroma Key Circuit, July 18, 1972. In the chroma key system a foreground camera may, for example, photograph a girl smoking a cigarette in front of a blue painted screen. The electronic chroma key device is capable of sensing specific colors and, in this case, would be adjusted to sense the blue of the background screen. The electronic signal thus derived from the blue screen portion of the foreground camera is utilized within the device to generate a "keying" signal which, in turn, is used in a television special effects amplifier to switch "off" the blue portions of the picture from the foreground camera, and simultaneously to switch "on" the background picture information generated by the second electronic camera. Because of the particular switching techniques used to combine the two camera signals, the system operates properly only when the blue background is uniformly lighted, and when edge transitions between the blue background and the foreground subject are particularly well defined. For this reason, the chroma key system does not sense fine details such as smoke, and it does not operate properly when transparent or semi-transparent materials appear with the subject in the foreground.
The relatively high brightness of the blue screen background used in traveling matte systems, combined with the optical flare introduced by all complex optical systems, particularly television zoom lenses, results in a false blue halo around objects appearing in front of the blue screen. A girl's black hair will, for example, have blue edges. A man's black suit will take a bluish cast.
In a system described in U.S. Pat. No. 3,778,542, Hanseman Blue Screen Traveling Matte System, Dec. 11, 1973, a subtraction concept rather than a switching technique is used to remove the blue background. The average of the red and green signals is derived and then electrically subtracted from the blue signal. The blue signal of the foreground camera as thus removed from the composite signal is replaced by signals from a background camera or other background source when it is desired to key the background image into the foreground image.
While traveling matte systems of the type described in the Hanseman patent provide certain improvements such as some reduction in the false blue halo and the ability to work with transparent objects in the images, such systems still suffer from a number of disadvantages. For one thing since it is virtually impossible to subtract all signals below the clipping level used, blue halo is still present in varying degrees. Of equal importance is the fact that such systems are still color dependent. Thus, where the keying color is blue, no other object in the foreground image may be blue in color. If individuals in the foreground image mistakenly wear blue clothing, for example, portions of the background image will be seen instead of the blue portions of their clothing.
Accordingly it would be advantageous to provide a keying system which is independent of color to the extent that any color can be used in both the foreground and background images. It would furthermore be advantageous to provide a keying system which eliminates the annoying blue halo effect. Still further advantages would be derived from a keying system which is relatively compact, lightweight, and which operates in a positive and dependable fashion, when compared with prior art systems.